1. Field of the Invention:
This invention relates to a voltage controlled oscillator, and more particularly to that having an oscillation frequency free from variation in resistance used therein.
2. Description of the Prior Art:
The voltage controlled oscillator is widely used in PLL (Phase Locked Loop) circuits and so on formed in a semiconductor integrated circuit. The PLL circuit has a wide application such as an FM stereo multiplexer. In the FM stereo multiplexer, a frequency signal of about 76 KHz is generated from a voltage controlled oscillator and is divided to provide a frequency signal of about 19 KHz, the phase of which is compared with a pilot signal of 19 KHz in an FM composite signal. The result of the comparison is derived as a form of voltage which is fed to the voltage controlled oscillator to control the oscillator to exactly oscillate with 76 KHz.
The voltage controlled oscillator in the prior art is composed of a C-R time constant circuit charged by a controlled current, a differential amplifier having a first input point receiving the voltage from the C-R time constant circuit, a second input point and a load of current mirror circuit, a first transistor receiving the output signal from the current mirror circuit at its base and feeding its collector output to the first input point of the differential amplifier, and a second transistor receiving the output signal from the current mirror circuit at its base and feeding its collector output to the second input point of the differential amplifier through a voltage divider of resistors. In a semiconductor integrated circuit, all the circuit elements except for the C-R time constant circuit are formed on a semiconductor chip. Especially, the resistances of the resistors in the voltage divider are sensitive to the change in manufacturing condition of the integrated circuit. The variation of the resistances affect the oscillation frequency of the voltage controlled oscillator in the prior art. Thus, the voltage controlled oscillator in the prior art has a drawback that a stable oscillation frequency irrespective of the manufacturing condition cannot be obtained if it is formed in a semiconductor integrated circuit.